fix(desktop): stop a stale composer model pinning every new chat

Settings → Model while a chat is open flips the composer source to
'default' but leaves the live session's model painted. Sending that
value on session.create pinned every new chat and skipped model.default.

Only a manual composer pick is a per-session override.

Co-authored-by: Tharanee <tharanee@tharanee.net>

apps/desktop/src/app/session/hooks/use-session-actions.test.tsx

@@ -60,6 +60,7 @@ import {
   setCurrentCwd,
   setCurrentFastMode,
   setCurrentModel,
+  setCurrentModelSource,
   setCurrentProvider,
   setCurrentReasoningEffort,
   setMessages,
@@ -567,6 +568,7 @@ describe('createBackendSessionForSend profile routing', () => {
     $currentFastMode.set(false)
     $currentModel.set('')
     $currentProvider.set('')
+    setCurrentModelSource('')
     $currentReasoningEffort.set('')
     setNewChatWorkspaceTarget(undefined)
     vi.restoreAllMocks()
@@ -609,6 +611,67 @@ describe('createBackendSessionForSend profile routing', () => {
     expect(params).toMatchObject({ source: 'desktop' })
   })
 
+  // Regression (Settings → Model doesn't stick): a stale composer selection
+  // must not be shipped as a per-session override on a NEW chat.
+  //
+  // Saving Settings → Model while a session is live deliberately leaves
+  // $currentModel painted with the LIVE agent's model (applySavedMainModel
+  // keeps the live session authoritative) and only flips the source to
+  // 'default'. If session.create still sent that value, every new chat was
+  // pinned to the old model and the backend never resolved model.default —
+  // the user-visible "my default won't change" bug.
+  it('omits a default-sourced selection so the backend resolves model.default', async () => {
+    const params = await createWith(() => {
+      // What the composer holds after Settings saved a new default while a
+      // chat was open: the previous session's model, marked default-sourced.
+      setCurrentModel('openai/gpt-5.6-sol')
+      setCurrentProvider('openai-codex')
+      setCurrentModelSource('default')
+    })
+
+    expect(params).not.toHaveProperty('model')
+    expect(params).not.toHaveProperty('provider')
+  })
+
+  it('still sends an explicit manual pick as a per-session override', async () => {
+    const params = await createWith(() => {
+      setCurrentModel('anthropic/claude-opus-5')
+      setCurrentProvider('anthropic')
+      setCurrentModelSource('manual')
+    })
+
+    expect(params).toMatchObject({
+      model: 'anthropic/claude-opus-5',
+      provider: 'anthropic'
+    })
+  })
+
+  // An unset source is the first-run/cleared state — nothing the user picked,
+  // so it must not pin the session either.
+  it('omits the model when no selection source is recorded', async () => {
+    const params = await createWith(() => {
+      setCurrentModel('openai/gpt-5.6-sol')
+      setCurrentProvider('openai-codex')
+      setCurrentModelSource('')
+    })
+
+    expect(params).not.toHaveProperty('model')
+    expect(params).not.toHaveProperty('provider')
+  })
+
+  // Effort and fast mode are independent of the model-override decision.
+  it('keeps sending reasoning effort even when the model is omitted', async () => {
+    const params = await createWith(() => {
+      setCurrentModel('openai/gpt-5.6-sol')
+      setCurrentProvider('openai-codex')
+      setCurrentModelSource('default')
+      setCurrentReasoningEffort('high')
+    })
+
+    expect(params).not.toHaveProperty('model')
+    expect(params).toMatchObject({ reasoning_effort: 'high' })
+  })
+
   it('passes the current workspace cwd into session.create', async () => {
     const params = await createWith(() => {
       $currentCwd.set('/remote/worktree')
@@ -658,6 +721,10 @@ describe('createBackendSessionForSend profile routing', () => {
 
     setCurrentModel('anthropic/claude-sonnet-4.6')
     setCurrentProvider('anthropic')
+    // A real composer pick marks the selection manual; this test drives the
+    // atoms directly, so set the source explicitly. Only a manual selection
+    // rides along as a per-session override.
+    setCurrentModelSource('manual')
     setCurrentReasoningEffort('high')
     setCurrentFastMode(false)
 

apps/desktop/src/app/session/hooks/use-session-actions/index.ts

@@ -71,6 +71,7 @@ import {
   $newChatWorkspaceTarget,
   $sessions,
   $yoloActive,
+  getCurrentModelSource,
   getSessionOwnerHint,
   type NewChatWorkspaceTarget,
   resolveComposerSessionKey,
@@ -242,8 +243,9 @@ function reconcileAuthoritativeMessages(
 // mode one backend serves every profile, so an omitted profile silently lands the
 // chat on the launch (default) profile — the "rubberbands back to default" bug.
 // A no-op for single-profile/local-pooled users (a backend resolves its own launch
-// profile to None). The sticky UI model/effort/fast ride as per-session overrides,
-// never the profile default (that lives in Settings → Model).
+// profile to None). Effort/fast still ride as per-session overrides. Model and
+// provider only ride when the composer source is 'manual' — a default-sourced
+// value is a mirror of Settings → Model and must not pin the new chat.
 async function desktopSessionCreateParams(
   cwd: string,
   capturedRoute = resolveNewChatOwnerRoute()
@@ -252,11 +254,17 @@ async function desktopSessionCreateParams(
   // profile handshake below can yield long enough for background config/model
   // refreshes to finish; reading atoms afterward would silently create the
   // session with a different selection than the one the user submitted.
+  // Settings → Model while a session is live leaves $currentModel painted with
+  // the live agent (applySavedMainModel) and only flips the source to 'default'.
+  // Shipping that stale value as an override pins every new chat to the old
+  // model. Omit model/provider unless the source is 'manual'.
+  const isManualSelection = getCurrentModelSource() === 'manual'
+
   const selection = {
     effort: $currentReasoningEffort.get().trim(),
     fast: $currentFastMode.get(),
-    model: $currentModel.get().trim(),
-    provider: $currentProvider.get().trim()
+    model: isManualSelection ? $currentModel.get().trim() : '',
+    provider: isManualSelection ? $currentProvider.get().trim() : ''
   }
 
   const profile = capturedRoute?.profile || $newChatProfile.get() || normalizeProfileKey($activeGatewayProfile.get())